GARNET, in mineralogy a closely related group of silicate minerals. The name is from Lat. granatum, a pomegranate, in allusion to the resemblance of the crystals to the seeds of this fruit in shape and colour. Garnets were worn as beads in ancient Egypt, and have been extensively used as gem stones. The mod em carbuncle is a deep-red garnet (almandine) cut en cabochon, or with a smooth convex surface frequently hollowed out at the back in consequence of the depth of colour, and sometimes en livened with a foil. Though not extensively employed by the Greeks as a material for engraved gems, it was much used by the Romans of the empire. Flat. polished slabs of almandine or "precious garnet" are found inlaid in mosaic work in Anglo-Saxon and Merovingian jewellery.
The garnets form a well defined group of orthosilicates of the general chemical formula 3R"O•R"',0,.3SiO, in which R"----Ca, Mg,Fe",Mn" and R"'=AI,Fe"',Cr"', while silicon in some varieties is partly replaced by titanium. The following pure species are recognized : Grossular, 3CaO.A1,O,,3Si02 j Pyrope, Almandine, 3FeO.A1,0,.3SiO2; Spessartine, Andradite, Uvarovite, 3CaO.Cr,O,.3Si0,. Melanite, Iwaarite and Schorlomite are vari eties of andradite containing significant percentages of titanium ; Mn"', Y, and Zr appear rarely as minor constituents. Most gar nets, however, prove on analysis not to be strictly any one of these minerals, but isomorphous mixtures of two or more of the end members, the particular R"or R"' being partly replaced by other metals of the same type.
All garnets crystallize with cubic symmetry, usually in rhombic dodecahedra (i 1o), or trapezohedra (211) or in a combination of the two. The hexoctahedron is found in some combinations, but the octahedron and cube forms are rare. An imperfect cleavage or parting parallel to the dodecahedron obtains, but is seldom observed in thin sections of garnet crystals. The hardness is variable, 6.5 to 7.5, the lime-alumina garnet being the softest. Density varies from 3.4 to 4.3 according to the composition. The refracting power is high, varying with the composition, thus :— The refraction of andradite increases with increase in titanium content, a schorlomite or melanite from Kuusamo, Finland, with 18.98% To, having a refractive index of 2•01.
Being cubic, garnets are normally singly refracting or isotropic, but the calcium garnets (grossular, andradite, melanite and uvarovite) are frequently birefringent, a fact which led Mallard to conclude that the garnets are really anorthic crystals with close approximation to cubic symmetry. Klein has referred the pseudo cubic garnets to four structural types in which uniaxial and biaxial subindividuals interpenetrate, forming the octahedral, dodecahedral, trapezohedral, and topazolitic structures, distin guished in polarized light by the manner in which the garnet crystals break up into doubly refracting sectors. No complete explanation of anisotropism in the garnet group of minerals is however yet available. Some of the zoned grossular and andradite garnets show a succession of isotropic and anisotropic shells.
Most of the garnets melt incongruently and break up into other compounds among which anorthite, monticellite, pyroxene, melilite, spinel and iron oxides have been recognized. From dry melts, spessartine and melanite have been crystallized while grossular has been synthesized under pressure by reaction of calcium orthosilicate and aluminium chloride. Ferriferous garnets fuse to a magnetic globule.
Almandine (precious garnet), the iron-aluminium garnet, de rives its name by corruption from alabandicus, the name given by Pliny to a stone found at Alabanda, a town in Asia Minor. It is usually of a deep red colour inclining to purple and shows a characteristic absorption spectrum consisting of three bands. The mineral is used considerably in jewellery. The home of almandine in igneous rocks is in the granite-gneisses particularly those of Archaean age, and in the dynamically metamorphosed argillaceous sediments—mica schists, para-gneisses and granulites. In areas of progressive dynamic metamorphism, the entry of almandine in these latter rocks marks a definite grade of metamor phosis, the mineral being generated from chlorite and quartz. Almandine frequently alters to chlorite and pseudomorphs of this mineral are common in mica schists. Noteworthy localities for large and well crystallized almandines are the schists of the St. Gotthard, the Zillerthal, and Fort Wrangell in Alaska.
Pyrope, the magnesium-aluminium garnet, is named from the Greek irupwiros (fiery eyed) in allusion to its deep red colour. Here are classed those garnets sometimes referred to as Bohemian garnet, Cape ruby and rhodolite. A pyrope of typical blood red colour is the common garnet of jewellery. It is distinguished from the red almandine by lower refraction and density. Pure pyrope is unknown in nature, the most magnesian type yet ex amined containing 75 molecular % of this constituent. The pure mineral would doubtless be colourless, the rich colour of the natur ally occurring pyrope being due to one or more of the contained metals iron, manganese or chromium. Dry melts of the corn position of pyrope crystallize at atmospheric pressure to an assemblage consisting of f orsterite, cordierite and spinel, and it is probable that pyrope is formed in nature only under high pressures. The magnesian garnets occur only in eclogites, perido tites and serpentines resulting from the alteration of olivine-rich rocks. Noteworthy localities for their occurrence are Zoblitz and Greifendorf in Saxony and Meronitz in Bohemia, where they are derived from serpentines. In North America they occur in peridotites in Kentucky, New Mexico and other localities. The Kimberlite pipes of the South African diamond fields contain in the "blue ground" irregular or rounded crystals of blood-red to brown pyrope (the so-called Cape ruby) examples of which have been found enclosing crystals of diamond. These garnets are primary crystals of an igneous eclogite (griquaite) or peridotite. Pyrope weathers usually to chlorite, but in the serpentines it is frequently found surrounded by a fibrous rim composed largely of amphibole, pyroxene and a spinellid mineral. This fibrous crust is frequently referred to as Kelyphite (Gr. KEXV4 OS a nut shell) . Though not always of the same constitution, a magnesian amphi bole is a common constituent together with spinel or picotite. In most cases it is probably a reaction rim due to magmatic resorp tion. The production of amphibole and spinel by mutual reaction of pyrope and olivinic liquid may be represented as follows: The resulting products have a distinctly greater molecular volume, and it is probable that a reaction of this type sets in during the intrusion of pyrope-bearing peridotites to higher levels in the crust.
Grossular, the lime-alumina garnet, is named from Lat. grossularia, a gooseberry, in allusion to the common pale green colour of its crystals. Here are classed the calcium garnets known as cinnamon stone, hessonite, romanzovite and succinite. When pure the mineral is colourless or white but it is frequently pale green, amber, red, or even emerald green from the presence of chromium. The red variety (cinnamon stone, chiefly from Ceylon) is often confused with zircon (hyacinth) from which it is readily distinguished by its much lower specific gravity. Gros sular melts incongruently and an assemblage consisting of anor thite, wollastonite and gehlenite is obtained from its dry melts. Typically a metamorphic mineral, it occurs only in unmetamor phosed igneous rocks when these have been contaminated by lime-rich inclusions. It occurs as a subordinate constituent of some saussurites and rarely as a metasomatic or pneumatolytic product in altered serpentines. The characteristic home of gros sular however is the thermally altered calcareous sediment where it accompanies other lime-rich minerals such as scapolite, idocrase and wollastonite. In regional metamorphism it is developed in similar rocks, but it is noteworthy that in these occurrences the garnets are frequently isotropic, unlike those of contact rocks. Noteworthy localities for its occurrence in fine crystals are in the Ala valley (Piedmont) where it occurs in hyacinthine dodecahe dra together with diopside and idocrase, an assemblage common in mineral collections, in contact limestones at Monzoni, in ejected limestone blocks at Vesuvius, and in Elba developed in yellow octahedra.
Spessartine, the manganese-aluminium garnet (from Spessart, Bavaria, where it occurs in red trapezohedra in granite) is usually of red, brownish-red or yellow colour. The crystal form is commonly trapezohedral (211) . This mineral, or a spessartine rich almandine, is of widespread distribution. It occurs in gran ites and pegmatites and as a pneumatolytic product in cavities or lithophysae of acid lavas. In hornfelses and crystalline schists derived from manganiferous, argillaceous and quartzose sedi ments it is a characteristic mineral. In the crystalline schists, in areas of progressive metamorphism spessartine may be generated at an early stage, entering into the constitution of phyllites be fore biotite is synthesized. Noteworthy localities for its occurrence are in granite at Aschaffenburg (Spessart), in the cavities of rhyolites near Simpson (Utah) and Nathrop (Colorado). In the whetstones of the Ardennes minute isotropic spessartine (or spessartine with considerable percentages of the grossular mole cule) forms colourless or reddish-yellow dodecahedra, often in great abundance. Fine large trapezohedra of spessartine are re corded from numerous localities in the central provinces of India, where they form constituents of important manganese ore deposits.
With andradite, calcium ferric garnet—named after J. B. d'Andrada who first examined it—are included the garnets known as allochroite, aplome, colophonite, demantoid, jelletite and topazolite. The common colour of andradite is brown, but green, yellow and wine colours are not infrequent. The grass green demantoid is used as a gemstone and possesses high refractive and dispersive power. Andradite is a typical metamorphic mineral, but is found also in igneous rocks which have assimilated frag ments of limestone. It is a characteristic constituent of andradite skarns, metasomatic rocks arising at the contacts of limestones and acid plutonic rocks such as granites and quartz diorites. The iron content of the andradite is largely provided by the solutions emanating from the cooling igneous intrusion. Noteworthy local ities for andradite in Europe are the contact aureoles of the Devonian igneous intrusions of the Oslo region, and Arendal. In North America andradite contact zones are frequently the home of important ore deposits of iron and copper, e.g., the copper ore occurrences of Conception del Oro (Mexico) and the Clifton Morenci district (Arizona). Melanite, Iwaarite and Schorlomite are titaniferous andradites of entirely different geological mode of occurrence. They are practically limited to intermediate and basic alkaline igneous rocks. These garnets are usually black, dull or resinous and in thin section dark brown, often zoned with shells of varying titanium content. In schorlomite titanium is present, not only replacing silicon but also as It occurs in the nepheline syenites of Magnet Cove (Arkansas) and in the leucitophyre of Horberig, Kaiserstuhl. The original Iwaarite found in the ijolite of Iwaara, Finland, contains as much as 25% Titanium rich melanites occur in the alkaline and ultra alkaline igneous rocks, nepheline syenites, ijolites, borolanites, leucitophyres, phonolites, etc., noteworthy localities for their oc currence being Loch Borolan (Assynt), the Kaiserstuhl, Fen dis trict (Norway), Kola Peninsula, Magnet Cove, Port Cygnet (Tasmania) and other alkaline provinces.
Uvarovite, named in honour of the Russian minister, Count Uvarov, is a rare emerald-green calcium-chromium garnet known in altered serpentines and metamorphosed limestones, in the first named rocks in cavities associated with chromite at Bisersk in the Northern Urals and also on Skyros and in limestones in Tasmania and at Orford (Canada). Crystals of uvarovite usually show anomalous birefringence.
An isomorphous mixture of grossular, almandine and pyrope forms the red garnet common in dynamically metamorphosed igneous rocks of the dolerite-gabbro group—amphibolites, horn blende schists and pyroxene rocks. The almandine molecule is dominant. In the amphibolites the garnets show a range of py rope content from 9-28 molecular %, and an average grossular content of 24%• (C. E. T.)